Fuel cells are power generation devices that utilize a power generation system based on electrochemical reactions, and have characteristics such as excellent power generation efficiency, environmental compatibility, and so forth. Such fuel cells are formed of a fuel electrode, which is a fuel-side electrode, an air electrode, which is an air (oxidant)-side electrode, and an electrolyte that is disposed between them and that allows only ions to pass therethrough, and various systems have been developed in accordance with types of electrolytes.
Among these, solid oxide fuel cells (hereinafter, referred to as “SOFC”) are fuel cells that employ ceramics, such as zirconia ceramics or the like, as an electrolyte and that are operated by using, as fuel, hydrogen and hydrocarbon-based gas, such as carbon monoxide, methane or the like, gas generated by means of equipment that gasifies carbonaceous raw materials such as coal or the like, city gas, natural gas, or mixed gas containing a plurality of these components. Such an SOFC enables power generation with high power generation efficiency, for example, by constructing a combined power generation system in combination with an internal combustion engine, such as a micro gas turbine (hereinafter, referred to as “MGT”) or the like.
In SOFCs, a technology has been under consideration in which, when power generation is stopped or tripped, a fuel-gas supplying system, an oxidizing-gas supplying system, a fuel-gas exhausting system, and an oxidizing-gas exhausting system are shut off, thus creating a state in which fuel gas and compressed oxidizing gas are contained in the SOFC system, and the SOFC is protected at this time by controlling the system pressure difference between the fuel system and the air system associated with stopping power generation by purging inert gas, such as nitrogen, onto the fuel electrode.
Patent Literature 1 described below discloses a technology used in a combined power generation system in which an SOFC and gas-turbine power generation equipment are combined for supplying pressure adjusting gas, such as reducing gas, nitrogen gas, or the like, in order to prevent a pressure drop associated with a temperature drop in the system in the case in which power generation is stopped, a fuel supplying system and an oxidizing-gas supplying system are shut off, and aeration gas is contained inside the SOFC system.
Patent Literature 2 described below discloses a technology used in a power generation system, having a vaporizer that vaporizes mixed liquid of fuel and water when the operation of power generating cells is stopped and a reformer that generates reformed gas containing hydrogen gas and carbon monoxide gas from the mixed gas, for adjusting internal pressures of individual channels, namely, an air channel, a fuel channel, an off-gas channel, and a water channel, by setting the vaporizer temperature at a higher temperature than the boiling point of the water and by making the water pass through the vaporizer to form water vapor.
Patent Literature 3 described below discloses a technology that maintains the internal pressures of fuel cells at atmospheric pressure by replenishing fuel gas, reducing gas, or inert gas.